The lawsuit, filed in federal court in San Francisco, demands that the government take action on a petition environmentalists filed earlier to have polar bears listed as "threatened" under the Endangered Species Act.

Once a species is listed as threatened, the government is barred from doing anything to jeopardize the animal's existence or its habitat. In the case of the polar bear, the environmentalists hope to force the government to curb U.S. emissions of greenhouse gases such as carbon dioxide.

The Center for Biological Diversity, the Natural Resources Defense Council and Greenpeace said extensive scientific evidence shows sea ice is melting because of global warming.

"Global warming and rising temperatures in the Arctic jeopardize the polar bear's very existence," said Melanie Duchin of Greenpeace. "Polar bears cannot survive without sea ice. Polar bears could disappear in our lifetime if we don't take action."

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Valerie Fellows, a Fish and Wildlife Service spokeswoman in Washington, said she did not have the lawsuit in front of her and could not comment on its specifics.

America's polar bears are found in Alaska. The Beaufort Sea stock off Alaska's northern coast is estimated at 2,000 animals. The Bering-Chukchi stock off Alaska's northwest coast, a population shared with Russia, is estimated at 2,000 to 5,000.

There is no firm count of polar bears, and the lawsuit did not indicate how many may have been lost because of retreating ice.

In September, the University of Colorado's National Snow and Ice Data Center, NASA and the University of Washington announced a "stunning reduction in Arctic sea ice at the end of the northern summer."